1. Field of the Invention
The present invention relates to high-performance and low-cost thermo-optic devices for use in wavelength division multiplexing (WDM) optical communication systems, more specifically to a digital thermo-optic switch and a tunable wavelength filter having an optical add/drop multiplexer (ADM) function which are superior in thermal stability, low in optical propagation loss at 1.55 to 1.58 .mu.m band, superior in wavelength selectivity, and low in production costs.
2. Description of the Related Art
As already known, a wavelength division multiplexing (WDM) optical communication system is a network system aiming for an increased transmission capacity by multiplexing wavelength different signals in the transparent wavelength in the transmittable wavelength band of the optical transmission path. The mainstream of optical communication in the future is being determined to be the wavelength division multiplexing optical communication. Recently, practical applications of lightwave communication technology begin to spread in areas of not only linking of signals, that is, transmission, but also in node of signals, that is, switching, and towards the area of signal processing.
Presently, in the U.S. and Europe, test beds of wavelength division multiplexing network are being carried out. In the test bed, strict brush-up is continued towards practical applications of various devices/components such as lasers, amplifiers, switches, filters, and wavelength converters. In the present stage, prospective candidates are not yet obtained other than in lasers and amplifiers. For example, speaking about switches, those which are operating in principles of electro-optic effect, thermo-optic effect, Stark effect, and acousto-optic effect are desperately competing. As to materials, various switches composed of ferroelectric crystals, glass, polymers, semiconductors, and the like are intensively competing. In the future, those parts which are superior in characteristics and have high practical applicabilities will be selected.
The above-described node for wavelength multiplexing specifically indicates optical path cross-connects for direct switching the optical path without O/E-E/O conversion, as is, or a wavelength selection gate, a so-called add/drop filter, which passes through a signal of a specific wavelength allocated per user.
As a selection switch which is the key of the cross connect system, not only of a high switching performance, one which has a simple control system, is wide in operation tolerance, high scalability and upgradability, and high in compactness is in demand in view of practical applications.
Further, the optical filter for achieving the optical add/drop function is required to be a device which is even higher in handling workability and costeffectivity than the optical path cross-connect switch since it is used in a hierarchy one step closer to the user than the optical path cross-connects in the network construction.
In the following, prior art of each of the above-described optical waveguide type thermo-optic switch, optical filter, and tunable wavelength filter will be described in further detail.
As the optical waveguide material, in the past, inorganic glass superior in transparency has been mainly used. However, inorganic glass has problems of being heavy and fragile, high in production cost, and the like, and recently, in place of inorganic glass, a movement is becoming active to use a transparent polymer having windows at optical communications wavelength regions (1.3 .mu.m and 1.55 .mu.m bands) in fabrication of single mode optical waveguide devices). Polymeric materials are easy for thin film formation by spin coating, dipping, or the like, and are thus suitable for producing large-area optical waveguides. Further, polymeric materials are high in potential to cost reduction as compared with glass-based and semiconductor-based optical waveguides, because the production is basically by a low-temperature process, development to reproduction is easy such as by mass production using embossing process. For such reasons, optical integrated circuits used in the area of optical communications and optical waveguide devices such as optical circuit boards used in the area of optical information processing are expected to be produced in large amounts and low costs using polymeric materials. Polymethylmethacrylate (PMMA) and other various transparent polymers are proposed, and research and development is intensively conducted in the production of optical waveguides using the polymers.
Yet further, of various optical waveguide devices, a digital thermo-optic switch comprising polymers in both core and cladding is an optical device sufficiently utilizing a superiority of polymer that the thermo-optic (TO) coefficient of polymeric materials is greater by a factor of 10 than inorganic glass material, which is actively being studied and developed in various organizations as a promising candidate of optical path switch used in optical path cross-connects and optical add/drop multiplexer in photonic transport networks. Recent advances are detailed in, for example, W. Horsthuis et al., ECOC95, pp. 1059-1062 (1995); N. Keil et al., Electron. Lett., vol. 31, pp. 403-404 (1995), Ooba et al., Proc. ACS PMSE, vol. 75, pp. 362-363 (1995), and the like. In particular, the polymeric digital thermo-optic switches reported by Horsthuis, Ooba et al., are greater in operation tolerance and smaller in wavelength dependence than the interference type phase difference switch by Keil et al., and are thus considered to be advantageous in practical application.
However, since digital type is in principle requires a large refractive index change compared to interference type, it is large in thermal load and has higher thermal resistance requirement for the material. For example, 1.times.2 thermo-optic switch module AK-SY1023SNC commercialized by Akzo Co. is specified to have operating temperatures of 17.degree. C. to 27.degree. C. for warranting stable operation of a cross-talk of -17 dB. Further, Ooba et al. report that in a thermo-optic switch using a PMMA-based polymer, reliability cannot be guaranteed because the waveguide temperature exceeds glass transition temperature specific to the material when applied power exceeds 130 mW at room temperature.
As described above, in consideration of device application of a polymeric optical waveguide, heat resistance has been regarded to be the most important problem, and recently, a material of improved heat resistance by containing an aromatic group such as benzene ring or using a silicone backbone (for example, those disclosed in Japanese Patent Application Laid-open No. 43423/1991, 328504/1992, and the like). However, introduction of such an aromatic group or a rigid main chain backbone is very effective for improving heat resistance, on the other hand, considerably tends to result in increased birefringence or degradation of film forming properties and processability (such as cracking). Therefore, no materials having well-balanced heat resistance, low birefringence, and film forming properties, and the like have heretofore been known. As described above, current polymeric digital thermo-optic switches yet have material dependent problems such as heat resistance or low propagation loss in 1.55 to 1.58 .mu.m band, and really practical devices have yet to be developed.
In the wavelength filter, when a polymeric material is used as described above, peak wavelength of the wavelength filter can be changed over a wide range utilizing the thermo-optic effect. FIG. 1 is a schematic diagram showing the structure of an example of wavelength filter using an arrayed-waveguide grating. In FIG. 1, reference numerals 1 and 2 indicate input and output waveguides, respectively, 3 and 4 are slab waveguides, and 5 is an arrayed waveguide comprising a plurality of channel waveguides having different optical path lengths. Light inputted from one port of the input waveguide 1 is diffracted by the slab waveguide 3 and distributed to respective channels of the arrayed waveguide 5. Light transmitting the arrayed waveguide 5, after given with a phase difference according to respective optical path length, is focused by the slab waveguide 4 and outputted from the output waveguide 2. At this moment, focused port differs according to the phase difference given in the arrayed waveguide 5. Therefore, only light in a specific wavelength band is transmitted and outputted from one port of the output waveguide 2. When light is inputted from the central port of the input waveguide 1, a peak wavelength .lambda.0 of light transmitted from the central port of the output waveguide 2 is given by the following formula: EQU .lambda.0=(n.sub.c .multidot..DELTA.L)/N
(wherein n.sub.c is an effective refractive index of each channel of the arrayed waveguide 5, .DELTA.L is an optical path length difference between respective channels, and N is an order of diffraction). From formula 1, it can be seen that peak transmission wavelength is proportional to the effective refractive index of the arrayed waveguide 5. Since polymeric material has large TO coefficient by a figure compared to inorganic glass material, a wavelength filter using a polymeric material, when the peak wavelength is varied utilizing the thermo-optic effect, can provide a tunable wavelength range wider by a figure compared to that using inorganic glass material. For example, in formula 1, when .DELTA.L=126 .mu.m, N=119, since temperature dependence of effective refractive index of PMMA-based polymer is 1.times.10.sup.-4 /.degree. C., the peak wavelength can be changed at a rate of -0.12 nm/.degree. C. Therefore, the peak wavelength can be changed by about 6.4 nm by a temperature change of about 55.degree. C. This peak wavelength change corresponds to a change of 800 GHz when converted to optical frequency of 1.55 .mu.m light. For the case of inorganic glass material, temperature change required for obtaining the above frequency change is about 550.degree. C., which is not practical. For the above reasons, an arrayed waveguide grating type wavelength filter having a wide tunable wavelength range has been produced using polymer waveguides and reported (Japanese Journal of Applied Physics, vol. 34, pp. 6416-6422 (1995)).
However, the above-reported wavelength filter using PMMA-based polymer had a problem of insufficient heat resistance when the peak wavelength is changed by temperature change, because glass transition temperature of the material is about 100.degree. C. Therefore, temperature variation range of this wavelength filter is limited to less than 80.degree. C. and, as a result, the tunable wavelength range is restricted. For example, in formula 1, when n.sub.c =1.46, .DELTA.L=126 .mu.m, N=119, free spectral range (FSR) of the wavelength filter is 12.9 nm, which corresponds to 1600 GHz, that is, 8 channels at 200 GHz intervals, or 16 channels at 100 GHz intervals. Here, FSR means a wavelength interval to the next diffraction order. That is, tunable wavelength range of the wavelength filer is limited to FSR. A variable wavelength range over the entire FSR is wide enough for tunable wavelength filters to select one of the all channels multiplexed in the FSR. However, since temperature dependence of an effective refractive index of the PMMA-based polymer is -1.1.times.10.sup.-4 /.degree. C., a peak wavelength variable range is 6.4 nm, which corresponds to temperature change of 55.degree. C., that is, temperature change from room temperature (25.degree. C.) to the upper limit (80.degree. C.) of usable temperature of PMMA. Then tuning of peak wavelength over the entire FSR has been difficult. Therefore, to achieve a wavelength filter of which the peak wavelength is variable over the entire FSR, development of a polymeric waveguide material of higher heat resistance has been required.
From the above reasons, recently materials of improved heat resistance by containing an aromatic group such as benzene ring or using a silicone backbone are reported (for example, Japanese Patent Application Laid-open Nos. 43423/1991, 328504/1992, and the like). However, such introduction of an aromatic group or a rigid main chain backbone is very effective for improving heat resistance, on the other hand, considerably tends to result in increased birefringence or degradation of film forming properties and processability (such as cracking). Therefore, no materials having well-balanced heat resistance, low birefringence, and film forming properties, and the like have heretofore been known. Therefore, current polymeric waveguide type wavelength filters have yet remain heat resistance problems of materials.
Still further, as wavelength filters having the add/drop multiplexing (ADM) function, there are an arrayed waveguide grating (AWG) filter by a silica planar lightwave circuit (PLC), a wavelength-fixed fiber grating filter, a tunable acousto-optic filter, a mechanical-driven type dielectric multilayer filter. Among these, the fiber grating is the most advanced in terms of performance and reliability, however, it has a major disadvantage in that it is not suitable for tuning and mass production. In the above-described wavelength division multiplexing (WDM) network system, an optical circuit for splitting optical signal (channel) of a specific wavelength from wavelength division multiplexing optical signals or for multiplexing the signal, that is, an add/drop multiplexer (ADM) system is required. Since the ADM system has a nature to be widely developed towards user systems and access systems, devices used therein are essentially required to be high in reliability and low in costs.